The present invention relates to a melt-extrudable thermoplastic composition and to the preparation of nonwoven webs therefrom.
Nonwoven webs are porous, textile-like materials which are composed primarily or entirely of fibers assembled in flat sheet form. The tensile properties of such webs may depend on frictional forces or on a film-forming polymeric additive functioning as a binder. All or some of the fibers may be welded to adjacent fibers by a solvent or by the application of heat and pressure. A nonwoven web may be reinforced with a scrim, gauze, netting, yarn, or other conventional sheet material. A nonwoven web may be incorporated as a component in a composite structure or laminate.
Nonwoven webs currently are employed in a variety of such disposable absorbent or protective products as diapers; incontinent products; feminine care products, such as tampons and sanitary napkins; wipes; towels; sterilization wraps; medical drapes, such as surgical drapes and related items; medical garments, such as hospital gowns, shoe covers, and the like; and industrial workwear, to name but a few. The nonwoven webs can be utilized as a single layer or as a component of a multilayered laminate or composite. When a multilayered laminate or composite is present, often each layer is a nonwoven web. Such multilayered structures are particularly useful for wipes, towels, industrial workwear, medical garments, medical drapes, and the like.
In order to improve the performance of a nonwoven-containing product, it sometimes is necessary to modify certain characteristics of the fibers of which the web is composed. A classic example is the modification of the hydrophobicity of polyolefin fibers by a topical treatment of the web with a surfactant or through the use of a melt additive.
Efforts also have been made to enhance or increase the tensile strength characteristics of nonwoven webs, particularly for such applications as sterilization wraps and industrial workwear. For example, U.S. Pat. No. 5,344,862 to Ronald S. Nohr and John G. MacDonald describes a melt additive system for thermoplastic polyolefins. The system includes two components. The first component is a polysiloxane polyether and the second component is a hydrophobic fumed silica. The two components are blended together before being added to the polymer. Melt extrusion of the resulting composition yields either nonwoven webs having significantly increased tensile strengths when compared to nonwoven webs prepared from the polymer alone or wettable webs requiring an amount of the first component which is significantly less than that required in the absence of the second component.
Another two-component system is described in U.S. application Ser. No. 07/958,630, which was filed on Oct. 9, 1992 in the names of Ronald S. Nohr and John G. MacDonald (see also PCT Application No. US93/09748, having International Publication No. WO 94/09066), now abandoned in favor of continuing application Ser. No. 08/382,835 which was filed on Feb. 3, 1995. The first component is the alkyl-substituted polysiloxane employed in the present invention and the second component is a hydrophobic fumed silica. The second component desirably is destructured in order to reduce the longest dimension of the silica particles to within a range of from about 0.001 to about 1 micrometer. When the additive system is mixed with a thermoplastic polyolefin, the resulting composition yields nonwoven webs having significantly increased tensile strengths when compared to nonwoven webs prepared from the polymer alone. The thermoplastic polyolefin may be a blend of two propylene polymers having different melt flow rates. For example, such blend may consist of from about 60 to about 40 percent by weight of a polypropylene having a melt flow rate of from about 30 to about 45 g/10 minutes and from about 40 to about 60 percent by weight of a polypropylene having a melt flow rate of from about 2 to about 20 g/10 minutes.
While either of the above-described additive systems is effective in increasing the tensile strength characteristics of nonwoven webs prepared by melt extruding compositions consisting of a thermoplastic polyolefin and an additive system, the two-component additive systems involve an additional processing step which requires considerable care in execution if the desired results are to be realized.